using System;

public class Program

{

    public static void Main()

    {

        double d1, d2, d3;

        d1 = double.Parse("0.00345");

        d2 = double.Parse("12654.004556");

        d3 = double.Parse("23.865000000000003");

​

        double x1 = RoundToSignificantDigits(d1, 3);

        double x2 = RoundToSignificantDigits(d2, 3);

        double x3 = RoundToSignificantDigits(d3, 3);

        Console.WriteLine(x1);

        Console.WriteLine(x2);

        Console.WriteLine(x3);

    }

    static double RoundToSignificantDigits(double d, int digits)

    {

        if(d == 0)

            return 0;

​

        double scale = Math.Pow(10, Math.Floor(Math.Log10(Math.Abs(d))) + 1);

        return scale * Math.Round(d / scale, digits);

    }

}

​